The present invention relates to rechargeable electrochemical energy storage systems, particularly to such systems, such as battery cells, comprising materials capable of reversibly taking up and releasing lithium ions as a means of storing and supplying electrical energy. More specifically, the invention relates to the formation and utilization of nanostructure transition metal fluoride:carbon amalgams, or nanoamalgams, as active electrode component materials in fabricating electrochemical cells, such as lithium battery cells, capable of exhibiting high specific capacity at high recharge rates.
During the course of development of rechargeable electrochemical cells, such as lithium and lithium-ion battery cells and the like, numerous materials capable of reversibly accommodating lithium ions have been investigated. Among these, occlusion and intercalation materials, such a carbonaceous and graphitic compounds, and transitions metal oxide spinels, have proved to be particularly well-suited to such applications. However, even while performing reasonably well in such recycling electrical storage systems of significant capacity, many of these materials exhibit detrimental properties, such as marginal environmental compatibility and safety, which detract from the ultimate acceptability of the rechargeable cell devices. In addition, some of the more promising materials are available only at costs which limit their widespread utilization.
Materials of choice in the fabrication of rechargeable battery cells, particularly highly desirable and broadly implemented Li-ion cells, have for some considerable time centered upon graphitic negative electrode compositions which provide respectable capacity levels in the range of 300 mAh/g. Complementary positive electrode materials in present cells comprise the less effective layered intercalation compounds, such as LiCoO2 which generally provides capacities in the range of 150 mAh/g. Alternative such intercalation materials, such as LiNiO2 and LiMn2O4, have more recently gained favor in the industry, since, although exhibiting no greater specific capacity, these compounds are available at lower cost and, further, provide a greater margin of environmental acceptability.
Due to the increasing demand for ever more compact electrical energy storage and delivery systems for all manner of advancing technologies, from biomedical to telecommunications, the search continues for battery cell materials capable, on the one hand, of providing greater specific capacity over wider ranges of cycling speeds, voltages, and operating temperatures while, on the other hand, presenting fewer environmental hazards and greater availability at lower processing and fabrication costs. Searches for more effective positive electrode materials in particular have become far-reaching with attention turning more frequently to the abundant lower toxicity transition metal compounds which are typically accessible at economical costs.
In this latter respect compounds of iron, e.g., iron oxides, attracted some past attention. However, although exhibiting electrochemical activity, iron oxides were found to function appropriately only at voltages which are too low for practical implementation in rechargeable lithium and lithium-ion battery cells. Upon further consideration of the economic advantages possibly attainable in transition metal compounds, interest shifted to examination of the more active fluoride compounds. Investigations into such use of these fluorides confirmed, however, that, while the open structures of the transition metal fluorides support the good ionic conductivity essential, in part, for useful electrode performance, the large band gap induced by the highly ionic character of the metal:halogen bond results in poor electronic conductivity in these materials. Without this latter essential conductive property to complement proven ionic conductivity, the transition metal fluorides were considered virtually useless as lithium battery electrode materials.
Despite the inconsequential performance of the transition metal fluorides in typical rechargeable cell fabrications, the theoretical promise of output voltages in the range of 3 V, due to the high ionicity of the compound bonds, prompted some further investigations into metal halides for use in electrode compositions. Arai et al., for example, in J. Power Sources, 68, 716-719 (1997), initially reported the use of iron trifluoride (FeF3) in an electrode composition comprising the admixture of about 25 parts acetylene black to 70 parts of FeF3 in an attempt to provide the necessary electronic conductivity. The performance of such a cell, despite the impractically low charge/discharge rate which extended over a 60 hour cycle period, was marginal at a discharge capacity over 4.5 to 2.0 V of only about 80 mAh/g vis-à-vis a theoretical (1e− transfer) capacity of 237 mAh/g. Subsequent independent fabrication and testing of similar battery cells at more realistic 4 hour cycle rates would yield no more than about 50 mAh/g.